It follows from [1]}, that for \(0 \le \alpha \le \beta \)  we have an embedding \(Z^{\beta } \subset Z^{\alpha }\) , and if \(\beta = 1\) , then \(D(\mathcal {A}) \subset Z^{\beta } \subset Z^{\alpha }\) . Hence
\(W_A \in D((-{\mathbb {A}})^{\alpha }).\) 
